Many contemporary aircrafts identify airports, and align with runways, through the use of an image correlation system. One such system which has been proposed for future aircrafts to identify airports and align with runways is the Autonomous Precision Approach and Landing System (APALS). APALS uses the aircraft's radar to “sense” (obtain an image of) the area around an airport. APALS then correlates the observed image with stored images.
Before correlating images, an APALS database is developed by sensing the ground around each airport and storing the sensed images in a database. When APALS is preparing to land at an airport, APALS takes an image of the ground around the aircraft. APALS then loads images from the database and correlates scenes along the approach path to determine the position of the aircraft. The location of the aircraft is determined through system knowledge of the coordinates of stored references in the images, and by determining an angular orientation and offset between the observed image and the expected (stored) image.
APALS and other similar systems, however, are dependent upon accuracy of the correlation process which analyzes the observed scene and the stored scene. This correlation process can create uncertainties, due to the potential confusion in which stored scene to apply. Confusion may occur, for example, because many scenes have similar appearances, which can cause some level of correlation with many scenes. Additionally, the actual scene may have changed since the stored image was taken, due to construction of new buildings, roads, or other landscape modifications. Further, if vehicles or other obstacles are accidentally positioned on a runway, the system may correlate incorrectly resulting in improper alignment and/or a failure to realize the presence of the obstacle. Weather can also make correlation between the observed image and the stored image difficult. For example, blowing sand, debris, or snow can make an observed image appear different than a stored image of the same area.
Other conventional systems rely on Global Positioning System (GPS) coordinates to identify the location of the aircraft. There are certain situations, however, in which GPS may not be reliable and in many cases independent validation is required. For example, ionospheric storms may alter the GPS signal, so as to make the signal non-reliable. To correct errors caused by ionospheric storms, some GPS systems have ground based signal correctors which calculate an error in the GPS signal. Air base GPS systems, however, may not be able to rely on ground based signal corrections, because of signal availability.
Uncertainties and errors are undesirable for aircrafts and aircraft landing systems, as it is imperative that the aircraft identify the correct area and avoid placing the aircraft in danger. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an apparatus and method for improving the recognition of the desired approach region to be used by an aircraft.